This invention relates to highly sensitive FM signal demodulation circuit for improving the threshold characteristic at a low reception the C/N ratio (carrier power to noise power ratio), and improving the noise characteristic of demodulated signals such as broad band TV-FM (television frequency modulated) signal waves, as well as a demodulation system therefor.
Heretofore, a frequency demodulation system using a frequency discriminator comprising a LC circuit or delay line has often been employed as the most convenient way for demodulating frequency--modulated signals. In this case, the S/N ratio (signal-to-noise ratio) of FM demodulated input signals relative to the C/N ratio of FM modulated input signals is expressed as: S/N=C/N .times.FI:constant) and thus the S/N ratio is in proportion to the C/N ratio. The C/N ratio is determined by the bandwidth B of a band pass filter used for restricting the bandwidth B of noises and signals upon demodulation. Usually, the bandwidth is represented according to the Curson rule as: B.apprxeq.2(.DELTA.f+fh) where .DELTA.f is the frequency deviation width of the FM modulated waves and fh is the highest modulated frequency in the modulated waves.
The relationship between the C/N and the S/N ratios changes linearly as long as the CN ratio is at a level of about 10 dB or higher but, if the C/N ratio decreases further, the S/N ratio is rapidly worsened by the impulse noises inherent to the FM demodulation. This particular C/N level is referred to as the threshold.
Generally, in the communication of transmitted TV signals, such as, satellite communication, FM modulation systems have often been used for the transmission of signals. In the satellite communication lines, the power margin upon reception is restricted as low as possible in view of the limit for the transmission power of a satellite, the stability in the satellite communication paths, the stability of the transmission power of the satellite, and the economy of the ground receiving facilities. The working point is often set near the threshold. Therefore, the received input is sometimes decreased due to the fluctuations in the circumstantial conditions, and the lower the receiving level is below the threshold, the more the quality of the demodulated images on a TV monitor is significantly disturbed by the impulse noises inherent to the FM transmission. This can even lead to a state where no modulated images can be obtained.
Accordingly, in the reception of a satellite communication, it is very important to improve the impulse noises by a means that is simple; in view of the improvement in the quality of the demodulated TV images, and the economy of the receiving facilities. In a convenient satellite-receiving device such as for the reception from a broadcasting satellite, it is important to obtain a simple method for improving the threshold characteristic.
One of the systems of improving the threshold characteristic relies on FM feedback. The present invention relates to such a system.
FIG. 1 shows the constitution of a FM feedback modulation system.
FM signals are inputted to an input terminal 1, they are frequency modulated in a frequency converter 2, then supplied by way of a band pass filter 3 to a frequency discriminator 4 that contains an amplifier and a limiter. FM demodulated signals are derived from an output terminal 8. A portion of the base band signals demodulated in the frequency discriminator 4 is supplied by way of a low pass filter 5 to a phase shifter 6 includes a feedback amplifier. The output from the phase shifter 6 is supplied to a variable frequency oscillator 7 and the output therefrom is supplied to the frequency converter.
The base band feedback circuit comprising the low pass filter 5, the phase shifter 6 and the like, when opened, has a similar circuit structure to that of the ordinary frequency modulation system only using only a discriminator 4, wherein the bandwidth B of the band pass filter 3 requires a Curson bandwidth determined by the highest modulated frequency and the frequency deviation of the FM signals as described above.
In the FM feedback modulation circuit illustrated in FIG. 1, if the oscillation frequency from the variable frequency oscillator 7 is controlled corresponding to instantaneous frequency changes in the FM signals inputted from the input terminal 1 and if it is exerted in the direction of compressing the frequency deviation in the output signals from the frequency converter 2 as compared with that of the input signals, it is possible to narrow the Curson bandwidth required for demodulation, that is, the bandwidth of the band pass filter 3. The C/N ratio of the FM signals applied to the discriminator 4 is improved as compared with the case where there is no feedback, and the threshold characteristic can be improved.
As is well-known, the television (color video) signals relevant to this invention comprise luminant signals and a color sub carrier component, and the base band signals thereof are very broad band signals up to 4.2 MHz in the NTSC system. Further, the magnitude of the frequency spectrum for the base band signals are significantly varied in the video signals depending on the kind of images (object) to be transmitted. Particularly, the amplitude of the color sub carrier component is changed greatly depending on the density (degree of saturation) of colors of the images. Because of the inherent nature of the TV signals, several difficulties have resulted from FM demodulation in the conventional system shown in FIG. 1.
Due to such a broad band characteristic of the base band signals, it is very difficult to supply the demodulated signal component from the discriminator 4 to the variable frequency oscillator 7 by way of a feedback circuit comprising the band pass filter 5, the phase shifter 6 including the amplifier, a connection circuit with the variable frequency oscillator 7 and other appended circuits at a high stability and a fidelity. Such stability and fidelity respect to the phase and the amplitude of this demodulated signal, would allow the oscillation frequency of the variable frequency oscillator 7 to vary at a determined relationship with the frequency change in the input FM signals.
Particularly, supplying of the broad base band signals to the variable frequency oscillator 7 by means of an active circuit involves difficult problems with regard to stability in view of the circuit structure as well as the correct application of the control signals thereto. If the oscillation of the variable frequency oscillator 7 does not exactly follow the modulated frequency components of great frequency deviation in the FM signals, the frequency deviation in the output signal is made larger as compared with that in the input signal of the frequency converter 2, whereby the FM signal components are eliminated by the band pass filter 3. This results in an adverse effect, in that the C/N ratio of the signals inputted to the discriminator 4 is lowered leading to a C/N level below the threshold.